As reported by BuzzFeed, the letter sent by Apple's General Counsel Bruce Sewell notes that Spotify has "benefited enormously" from Apple's App Store and asks the company to update its app to bring it in line with the developer guidelines.

Spotify, though, was furious at the initial update block, calling Apple's behaviour "anticompetitive" and suggesting that the company uses the App Store approval process as a weapon to harm competitors.

While the next stage in this battle is unclear, Apple appears to be implying that the Spotify app will not be updated until it falls into line.